The present invention relates to the field of imaging and processing microscopic features using beams of photons or charged particles.
Beams of charged particles or photons are widely used to view and process extremely small structures. Instruments used to produce images of microscopic structures include scanning electron microscopes (SEMs), scanning transmission electron microscopes (STEMs), scanning optical instruments, and focused ion beam (FIB) systems.
In these instruments, photons or charged particles, such as electrons or ions, are directed at a specimen, and secondary particles emitted from the surface or backscattered particles from the primary beam are collected. The type, energy, or quantity of secondary or backscattered particles can be determined to infer information about the surface points impacted by the beam. Images can be formed to represent the spatial distribution of certain properties or qualities, such as composition and topography, of the material surface or sub-surface.
When acquiring an image, the primary beam is scanned across the surface in a raster pattern, that is, the top row is scanned from a first edge and then each subsequent row is scanned in the same direction, with the beam moving rapidly back to the first edge after completing each row so that the beam will be in position to start the next row. By scanning juxtaposed rectangles of different sizes, it is possible to approximate irregular exposure patterns.
Information received from the detectors typically must be processed to create signals that are useful for controlling a display, such as a CRT-type display or flat-panel-type display. Processing steps may include amplification and further signal conditioning to enable digital storage of the processed signal data. Additional processing may be performed on the stored data for image enhancement prior to or subsequent to display. In scanning electron microscopes and scanning ion microscopes, for example, image data can be collected and displayed, with or without processing, in real time. The scanning of the beam in the display CRT must be coordinated with the scanning of the imaging instrument, so that the points on the image correspond in position with the points on the specimen.
Focused ion beam (FIB) systems are widely used in microscopic-scale manufacturing operations because of their ability to image, etch, mill, deposit, and analyze with great precision. Ion columns on FIB systems using gallium liquid metal ion sources (LMIS), for example, can provide five to seven nanometer lateral imaging resolution. Focused ion beams mill by sputtering, that is, physically knocking atoms and molecules from the specimen surface. Because of their versatility and precision, FIB systems have gained universal acceptance in the integrated circuit (IC) industry as necessary analytical tools for use in process development, failure analysis, and most recently, defect characterization. FIB systems are used in a variety of other micromachining operations including the fabrication of thin film magnetic heads for reading and writing information to data storage media, and micro electromechanical systems (MEMS).
The ion beam typically scans the surface of the specimen in a raster pattern and secondary electrons are collected to form an image of the specimen surface. Secondary ions can be collected and analyzed to form a map showing the composition of the surface. This map, or image, can be used to determine the feature to be milled. The raster pattern is reduced and moved to the feature to be milled, and the ion beam current is increased to mill the surface. A gaseous material is often directed to the sample at the impact point of the ion beam, and the ions induce a chemical reaction that selectively increases the etch rate or deposits material, depending on the gaseous compound that is used.
Any time the ion beam impacts the surface, atoms are sputtered from the surface. Although this is the desired result in a milling operation, when the ion beam is used for imaging, the sputtering causes unintentional surface damage. While the damage can be reduced by reducing the number of ions that hit the surface during imaging, for example, by reducing the beam current or by increasing the speed at which the beam traverses the specimen, some damage is unavoidable.
Raster scanning always covers a rectangular area. When the area of interest is non-rectangular, it is an unavoidable consequence of the raster scan pattern that some material outside the area of interest will also be subject to unwanted exposure and consequent damage of material. Therefore, raster scanning to produce images results in particle beam-induced irradiation damage or unwanted alteration.
One method of for reducing damage when using a raster pattern is described in U.S. Pat. No. 5,616,921 to Talbot et al. for xe2x80x9cSelf Masking FIB Milling.xe2x80x9d The system of Talbot et al. uses a raster pattern to scan a focused ion beam. A xe2x80x9cmask imagexe2x80x9d is created by comparing the image of points to a threshold value. When the beam is rastered to points exceeding the threshold value, the beam is attenuated, for example, by blanking or defocusing.
During milling operations, the beam is sometimes scanned in an arbitrary pattern, rather than in a raster pattern. A pattern generator implemented in hardware or software provides a sequence of points to the beam controller, which moves the beam from one arbitrary pixel to another. (Although the term xe2x80x9cpixelxe2x80x9d originally refereed to a xe2x80x9cpicture elementxe2x80x9d on a display, it is also used to indicate a xe2x80x9cdwell pointxe2x80x9d on a work piece surface, that is, a point to which the beam is directed and which is often represented by a single pixel on a display showing the work piece.) Arbitrary patterns may be generated which move the beam over the material surface in a manner which confines the beam to the area of interest and eliminates damage or alteration to surrounding areas. The beam is thereby confined within an area of interest defined by the pattern generator and performs its function to produce the desired material alteration within it.
However, while prior art pattern generation methods confine the desired material alteration to the area of interest during milling, it is necessary to examine or image the material thus exposed to determine if further material processing is necessary. Often the steps of processing and then reimaging the surface must be performed several times before the process is complete. At each step, it is required to switch from a generated pattern to a raster pattern to form an image of the specimen. Switching not only takes time, which reduces the productivity of the operation, using a raster pattern for images results in undesirable exposure of areas of the specimen that are not being processed.
An object of the invention is to enhance the use of systems using beams of photons or charged particles to image or process microscopic structures.
The present invention comprises a method and apparatus for using beams to process material. In accordance with one aspect of the invention, image information is collected as the beam is directed in an arbitrary pattern of points on the specimen. By collecting image information as the beam is directed in an arbitrary pattern, beam-induced damage to the surfacexe2x80x94which would occur if a raster pattern were usedxe2x80x94is avoided.
In accordance with another aspect of the invention, image information is collected as a beam is processing material, and a separate imaging step is not required. The invention thus permits xe2x80x9creal timexe2x80x9d imaging or data collection during processing. Collecting information as the material is being processed allows for xe2x80x9cclosed loopxe2x80x9d processing, in which the results of the processing are available in real time for controlling further processing. Overprocessing can be avoided and the time required to switch between an imaging mode and a processing mode is eliminated, thereby increasing productivity.
In another aspect of the invention, the delay between instructions to move the beam to a point and the time at which a signal is detected from the surface at that point is determined. At the high scan rates used in modern scanning machines, the delay time can be many dwell periods, which would make the image out of synchronization with the beam. By calibrating the delay for particular operating parameters, an accurate image and placement of beam is maintained.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.